Integrated services digital network (ISDN) communication systems enable telephone service providers to supply multiple types of signalling channels from a central office to a network termination interface at a customer premises site. An example of a reduced complexity `extended distance` ISDN communication network architecture is diagrammatically illustrated in FIG. 1, as comprising a PCM communication link (such as a T1 data rate (1.544 Mb/s) optical fiber link) 10, through which a central office (CO) 20 at a `west` end of the PCM link 10 transmits and receives signalling traffic with respect to a customer premises equipment (CPE) served by a channel bank 30 at a remote or `east` end of the PCM link 10. The central office 20 includes a central office switch 21 (such as a 5ESS switch manufactured by AT&T), that contains a plurality of line termination circuits (or line cards) 22, each of which is coupled over a local loop (twisted tip/ring pair) to local customer site.
As shown in FIG. 2, in order to provide service to remote customers, the channel bank 30 at the `east` end of the PCM link 10 terminates the link by way of a line interface unit (LIU) 31, which is coupled over an internal PCM bus 32 to a plurality of U-BRITE circuit cards 33 installed in the channel unit's backplane 34. Each respective U-BRITE circuit card 33 is dedicated to providing extended ISDN service to remote customer premises equipment via a local loop 35 between the U-BRITE circuit 33 card and digital communication equipment 40 installed at a respective customer premises.
A carrier system transceiver within the line interface unit 31 is operative, under control of an attendant communications control processor, to transmit and receive standard 2B+D ISDN data traffic over the PCM digital data link 10. To interface digital subscriber loop (DSL) over the local loop (twisted pair) 35 to the customer premises equipment (CPE) 40, the U-BRITE circuit card 33 includes a line transceiver and an associated line interface, which are also operative, under microprocessor control, to interface PCM data with the line interface unit 31 and to transmit and receive basic rate 2B1Q ISDN signals over the local loop to and from CPE 40.
Because each U-BRITE card's microcontroller (and associated memory) constitutes a substantial portion of the total cost of the U-BRITE card, ISDN equipment vendors have sought ways to simplify the complexity of the U-BRITE circuitry. (The historical reason for each U-BRITE card having its own processor is the fact that channel banks originally did not contain the requisite digital signal processing capability, making it necessary to install a processor on each digital communication card.)
One proposal to eliminate this hardware and cost penalty involves installing a plurality of U-BRITE circuits (e.g., eight such circuits) on a single `multi` U-BRITE circuit card, and controlling each of the card's U-BRITE circuits by a shared micro-controller installed on the same one card with the multiple U-BRITE circuits. While such a multi-circuit configuration offers a reduction in the number of components (processors) and therefore a cheaper card, it suffers from a major drawback in the event it becomes necessary to remove the card from the channel bank's backplane, for example, for the purpose of maintenance or replacement of an individual U-BRITE circuit. Since a multi-U-BRITE circuit card contains plural U-BRITE circuits, each associated with a respectively different customer, removal of the card will necessarily interrupt service to customers other than just the one connected to the U-BRITE circuit of interest.